Fiber-reinforced resin composites have gained an increasing market share for high performance parts used in various industries, as for example automotive and aircraft components. In the composite industry, continuous carbon and glass fibers are impregnated with polymeric resins, then wound or layered against rigid mold surfaces and cured into structural parts.
Two classes of resins are used in composites. A thermosetting resin reacts in the presence of heat or a catalyst to produce a 3-dimensional structure that cannot be reshaped. Epoxies and certain polyimides are examples of thermosetting resins used in composite technology. A thermoplastic resin retains its two-dimensional structure in the presence of heat and pressure. As a result, thermoplastic resins can be softened or melted many times, thus allowing for re-shaping. Polyether-etherketone (PEEK), ultra high molecular weight polyethylene (UHMWPE) and polybenzimidozole (PBI) are examples of high performance thermoplastics used in forming composites. In general, the art of forming high performance thermosetting resin composites is more advanced than the art of forming high performance thermoplastic composites.
A problem encountered in the preparation of fiber reinforced thermoplastic composites is that it is difficult to obtain good wetting of the reinforcing fibers or filaments, and consequently it is difficult to obtain composites in which air bubbles are absent at the interfaces between resin and reinforcing fiber. As a consequence, it is desirable to produce pre-pregs of fiber-reinforced resins in which the fibers constitute a relatively high percentage of the weight of the pre-preg and then incorporate this pre-preg into the resinous composite.
Various methods of coating fibers with the desired coating polymer are known. Basically, these are as follows:
1. Solution coating, i.e. dissolving the polymer in a solvent, coating the resulting solution onto a fiber tow, evaporating away the solvent, leaving the polymer as a coating on the fiber surface.
2. Suspension or emulsion coating, i.e. suspending or emulsifying a polymer powder in a non-solvent, coating the fiber tow with the resulting liquid suspension or emulsion, then evaporating the non-solvent carrier, leaving the polymer powder in intimate contact with the fibers. An additional thermal process step, to fuse the powder on to the fiber surface, is usually required.
3. Melt impregnation of a fiber tow with molten polymer.
4. Spinning processes, i.e., spinning a fiber of the polymer, weaving or commingling these fibers with the tow, then heating the fiber structure to melt the polymer fibers on to the adjacent reinforcing fibers.
Each one of these processes has one or more technical limitations. Solvents can be toxic or noxious, and in any case, are very difficult to remove from most high performance polymers. Solvents trapped in the final composite product can result in porosity and weak areas in the structure.
High polymer viscosity inhibits resin impregnation of the reinforcing fiber bundle with molten polymer.
Suspensions and emulsions usually require the addition of emulsifiers or suspending agents, which remain in the finished composite and which can adversely affect mechanical performance.
Commingling requires production of a polymeric fiber from the matrix material. This can be difficult to do and can be quite expensive.
Another coating process is electrostatic coating. Basically, charged particles of a desired coating agent are applied to the surface of a substrate which is either oppositely charged or grounded. While electrostatic coating techniques have been used to coat flat surfaces, e.g. sheets, and single rods or wires, only very recently (in 1989) has the use of dry powder coating techniques for coating of reinforcing fiber tows been reported in the literature.
Muzzy et al, in a publication, i.e. 34th International SAMPE Symposium, May 8-11, 1989, pages 1940-1951, describes preparation of thermoplastic tow pregs by electrostatic deposition of charged fluidized thermoplastic polymer particles directly from a fluidized bed on to a spread continuous carbon fiber tow. The tow, which is grounded, is passed through a fluidized bed of electrostatically charged polymer particles. The polymer particles are both electrostatically charged and fluidized by ionizing (or charging) a flowing air stream and then passing this air stream upwardly through a bed of polymer particles. The tow with particles deposited thereon is then heated in an oven to cause the polymer particles to melt and coat the tow fibers.
A disadvantage of the Muzzy et al process is that it works best with comparatively coarse polymer particles, e.g. those having average sizes of about 80 microns or larger, and does not work particularly well with finer particles. Better results in ionizing of air and charging of polymer particles are obtained at comparatively high air velocities than at lower air velocities. On the other hand, these comparatively high air velocities correspond to the fluidizing velocities of comparatively large particles (say about 80 microns or larger); the fluidizing velocities for particles approximately 5 to 10 microns in diameters i.e. in the same range as the fiber diameter, are well below those required for optimum air ionization and polymer particle charging.
Other electrostatic coating processes, such as the process shown and described in U.S. Pat. No. 4,084,109 to Christ et al, are also known.
U. S. Pat. No. 3,873,389 to Daniels describes a process and apparatus for pneumatically spreading thin carbon filaments from a tow bundle to form a sheet or tape in which the individual filaments are parallel. This is achieved by passing the tow continuously through a pair of slot venturi spreaders, with air flowing concurrently with the tow in the first and countercurrently to the tow in the second. The spread tow may then be continuously passed through a bath of impregnating material, which is usually a plastic resin of the epoxy, phenolic or polyimide type.
Cross, in Chemistry in Britain, Vol. 17, no. 1, pages 24-26, (1981), discloses several electrostatic powder coating processes for depositing dry charged paint, resin or pigment particles onto a grounded workpiece. In one embodiment (FIG. 1), fluidized powder particles are drawn into an electrostatic gun in which the particles are charged. The charged particles are sprayed onto the grounded workpiece.